Fire refractory ceramic moulded piece, use thereof and composition for production of moulded pieces

ABSTRACT

The invention relates to a fired refractory ceramic molded piece with a spinel matrix based on (Mg) 2+  (Al, Cr) 2   3+ O 4 , in which coarser particles based on chromium corundum and/or corundum and coarser particles based on ZrO 2  are present.

DESCRIPTION

[0001] The invention concerns a fired refractory ceramic molded piece, apossible use, as well as a composition for the production of said moldedpiece.

[0002] From Schulle “Refractory Materials”, 1^(st) Edition, 1990 (ISBN3-342-00306-5), pp. 288, 289, refractory materials based on chromiumcorundum, with a proportion of around 10% by weight CrO₂ and about 90%by weight Al₂O₃ are known. Also mentioned there are refractory materialsbased on Cr₂O₃/ZrO₂. At the same time, it is also noted that the use ofmaterials of this type as refractory ceramics is little known.

[0003] In DE 197 27 917 C1 is described a composition of 5 to 35% byweight Cr₂O₃ with a grain size <0.1 mm, 1 to 10% by weight ZrO₂ with agrain size between 0.5 and 4 mm, as well as 55 to 94% by weight Al₂O₃with a grain size between 0.2 and 4.0 mm for the manufacture of firedrefractory ceramic molded pieces that can be used in slag-carryingfurnaces with great thermal shock resistance and high operatingtemperatures, for example in rotary furnaces for burning garbage. Goodstructural elasticity and good corrosion-resistance are reported ofthese bricks.

[0004] However, in incinerator plants for special waste there is alwaysa high degree of corrosion of the matrix of the refractory material.This is true particularly for strongly alkaline slags, or for slags witha high iron oxide content, or with slags whose C/S ratio (basicity) isvariable during the respective process. Here, even bricks burned from acomposition in accordance with DE 197 27 917 C1 may reach their limits,in other words, exhibit unsatisfactory wear characteristics.

[0005] The objective of the invention is to offer molded pieces that, incontrast to products according to DE 197 27 917 C1 exhibit bettercorrosion resistance, especially improved corrosion resistance withrespect to slag with variable/changing basicity.

[0006] Application areas for such pieces may be: linings in wasteincinerator plants and other furnaces/melting equipment, or ingasification plants, in which aggressive melts and/or slags areencountered.

[0007] The basic idea of the invention is, in particular, to fashion thematrix of the molded piece to be more stable (more resistant) in theface of an aggressive slag.

[0008] A fired refractory ceramic molded piece according to theinvention is distinguished in its most general embodiment by a spinelmixture matrix, in which relatively coarse grains based on corundumand/or chromium corundum, as well as ZrO₂ are present. The desiredproduct properties are achieved, for example, by the simultaneouspresence of chromium corundum and zirconium corundum, or by thesimultaneous presence of corundum and zirconium mullite.

[0009] The spinel mixture matrix can be produced from appropriatecomposition components such as Cr₂O₃, MgO, or Al₂O₃ as individualcomponents and/or in the form of pre-synthesized spinels, in particularMA-spinels (MgO—Al₂O₃) or MCA-spinels, in particular (MgO—Cr₂O₃—Al₂O₃),or by other means.

[0010] The chromium corundum grains have their origin in the compositionin the oxides of chromium and aluminum, and are created during thefiring process. They may also be introduced, at least partially, aspreviously reacted components.

[0011] The ZrO₂-containing grains may be derived frombaddeleyite-containing material (raw material), or, for example, fromsinter-zirconium mullite, melt zirconium mullite, sinter zirconiumcorundum, melt zirconium corundum, or from a ZAC material (pre-reactedmaterial of SiO₂, ZrO₂ and Al₂O₃), separately or in a mixture.

[0012] A composition component based on an MA-spinel [(MgO,Al₂O₃)]-spinel, in a fraction of <0.5 or <0.3 or <0.1 mm may be used.This spinel, which preferably is not stoichiometiric, say, for example,with an excess of MgO, reacts with the chromium oxide of the compositionto form a mixed spinel in the form of (Mg)²⁺ (Al, Cr)₂ ³⁺O₄ and formsessential positions of the base material of the matrix of the firedproduct. This mixed spinel exhibits an outstanding resistance relativeto the named critical slags.

[0013] Aluminum oxide, especially as a component in the grain-size rangeof 0.5-10 mm, can react with (fine-particled) chromium oxide, and leadsto the already mentioned chromium corundum grains, which are distributedwithin the matrix the same as the said ZrO₂-containing grains.

[0014] It is self-evident that, depending on the degree of contaminationof the raw materials, additional ingredients may be present in thecomposition components, and thus become secondary constituents of themolded piece.

[0015] The aforementioned coarse grains in the fired product may attainan average diameter (d_(0.50)) between 0.5 to 10 mm.

[0016] The chromium oxide concentration within the chromium corundumgrains can fluctuate, depending on the firing temperature and furnaceatmosphere, as well as on the proportions of chromium oxide or corundum(Al₂O₃), while the firing process can be managed in a way such thatwithin the chromium corundum grains there remains at least one zone,especially a core zone, that is free of chromium oxide.

[0017] Correspondingly, the firing process can be controlled so that thechromium grains have their highest chromium oxide concentration withinthe region close to their surface.

[0018] FIGS. 1 to 4 show microscopic photographs on the indicated scaleof ground surfaces of a fired product. In them, the numbers signify: 1.Chromium corundum grains; 2. Zirconium corundum grains; 3. Mixed spinelmatrix; 4. Pores. The coarse grains based on chromium corundum andzirconium corundum are distinctly visible in the fine-grained spinelmatrix.

[0019] The matrix component, i.e. the proportion consisting essentiallyof the named mixed spinel, may constitute between 10 and 70% by weightof the total mass, and usually between 15 and 50% by weight.

[0020] By virtue of its outstanding stability, particularly vis-à-visaggressive slags, such as alkali-containing slags, or slags with a highiron oxide content, the fired refractory ceramic molded piece issuitable for the lining of a furnace for burning refuse, for example forthe incineration of special refuse, where slags of this type occur.

[0021] The composition (batch) for producing the previously named moldedpiece encompasses, in its most generalized implementing form, thefollowing components (which, alone, or with additional components, addup to 100% by weight):

[0022] 2 to 35% by weight chromium oxide <0.5 mm,

[0023] 1 to 15% by weight zirconium dioxide >0.2 mm,

[0024] 1 to 12% by weight magnesium oxide <0.5 mm,

[0025] 38 to 96% by weight aluminum oxide <10 mm.

[0026] The components being added to the composition may be pure oxidesor materials that introduce the named oxides into the composition in thestated proportions.

[0027] A particularly conspicuous aspect of this composition processlies in the fact that the composition, using exclusively a temporarybinding agent, e.g. sulfite waste liquor, can be manufactured intomolded pieces, firing them subsequently, at which time the sulfite wasteliquor is to almost entirely burned off.

[0028] By dint of the invention, that is, the use of, e.g.phosphate-containing binding agents can be avoided, which represents anadditional substantial advantage vis-à-vis the state of the art.

[0029] Contrariwise, the composition according to DE 197 27 917 C1cannot be processed with an exclusively temporary binding agent such assulfite waste liquor. With the known composition, it is impossible witha temporary binding agent to achieve an adequate degree of firmness inthe pre-firing state.

[0030] The composition may be varied within the scope of the followingprovisions:

[0031] The grain-size fraction of the Al₂O₃ component may be <6 mm or <4mm,

[0032] The grain-size fraction of the ZrO₂-containing component can bebetween 1 and 4 mm,

[0033] The grain-size fractions of Cr₂O₃ and MgO may be <0.2 mm, <0.1mm, or also >0.05 mm,

[0034] Al₂O₃, Cr₂O₃ and/or MgO may be introduced, first of all, at leastpartially as spinel, particularly MA- or MCA spinel, at 5 to 25% byweight, for example,

[0035] Where MgO is present at least partially as spinel, the Al₂O₃ orCr₂O₃ component needs to be reduced proportionally,

[0036] The content of Cr₂O₃ may be set at 5 to 25% by weight,

[0037] The ZrO₂ content in the composition may be 1 to 6% by weight,

[0038] The Al₂O₃ component in the composition may be between 64 and 87%by weight of the total composition, and a portion may be present in afine fraction (<0.1 mm),

[0039] The SiO₂ content should be limited to <3.0% by weight or, better:<0.1% by weight.

[0040] In this connection, it is recommended that a ZrO₂-containingcomponent be used which, if possible, is free of SiO₂ content,

[0041] The Fe₂O₃ content may be <0.5% by weight,

[0042] The MgO content in the composition may amount 3 to 8% by weight,

[0043] The Al₂O₃ content may be added at least in part as chromiumcorundum,

[0044] The MA-spinel can, by way of example, consist of the followingcomponents (all data are in % by weight): MgO: 31-35 Al₂O₃: 64-67 CaO:0.3-0.6 Fe₂O₃: 0.1-0.3

[0045] Besides other components such as Na₂O, SiO₂, whose respectiveproportion amounts to <0.2% by weight.

[0046] The composition, including its temporary binding agent, isprepared in the usual manner, and fabricated into molded pieces,principally by pressing. It is subsequently fired at temperaturesbetween 1,200 and 1700° C.

[0047] Starting with a composition of:

[0048] 8% by weight chromium oxide <0.1 mm

[0049] 5% by weight zirconium corundum 0.5 to 4 mm

[0050] 15% by weight non-stoichiometric MA-spinel (of previouslymentioned type) <0.1 mm

[0051] Remainder corundum <4 mm,

[0052] and adding 3% by weight sulfite waste liquor (relative to thetotal of components), bricks were pressed at 100 MPa pressure, and firedat 1,550° C. The bricks exhibited the following characteristic values:Rough density (per DIN EN 993-1): 3.35 g/cm³ Open porosity (per DIN EN993-1): 15.1% Cold compression strength (per DIN 51607): 170 N/mm²Softening under pressure (per DIN 51053 T1): T₀ > 1,700° C. Thermalshock resistance (per DIN EN 993, >30 cycles. Part 11, relative towater):

[0053] These values fall within the range of bricks according to DE 19727 917 C1. However, the bricks according to the invention prove to bedistinctly superior in their corrosion resistance, particularlyvis-à-vis the cited critical slags. Their resistance in a laboratorytest (slagging test in an induction crucible furnace) was 30% higher.

1. Fired refractory ceramic molded piece with a spinel matrix based on(Mg)²⁺ (Al, Cr)₂ ³⁺O₄, in which coarser particles based on chromiumcorundum and/or corundum and coarser particles based on ZrO₂ arepresent.
 2. Molded piece according to claim 1, in which the coarserparticles vis-à-vis the particles of the spinel matrix consist of thecomponents chromium corundum and zirconium corundum.
 3. Molded pieceaccording to claim 1, in which the corundum and/or chromium corundumgrains have an average diameter (d_(0.50)) of 0.5 to 10 mm.
 4. Moldedpiece according to claim 1, in which the ZrO₂-containing grains have anaverage diameter (d_(.50)) of 0.5 to 10 mm.
 5. Molded piece according toclaim 1, in which the spinel matrix constitutes 10 to 70% by weight ofthe total mass.
 6. Use of the molded piece in accordance with one of theclaims 1 to 5 for lining a furnace for burning refuse, particularlyspecial type refuse.
 7. Composition (batch) for the manufacture of amolded piece according to one of the claims 1 to 5, with 7.1 2 to 35% byweight Cr₂O₃ in a grain size fraction <0.5 mm, 7.2 1 to 15% by weightZrO₂, in a grain-size fraction >or equal to 0.2 mm, 7.3 1 to 12% byweight MgO in a grain-size fraction of <0.5 mm, 7.4 38 to 96% by weightAl₂O₃ in a grain-size fraction of <10 mm.
 8. Composition in accordancewith claim 7, in which at least a part of the components MgO, Al₂O₃, orCr₂O₃ is present as a spinel.
 9. Composition according to claim 8, inwhich at least a part of the components is present as MA- or MCA-spinel.10. Composition according to claim 7, in which the Cr₂O₃ content is 5 to25% by weight.
 11. Composition according to claim 7, in which theZrO₂-content is between 1 and 6% by weight.
 12. Composition according toclaim 11, in which the Al₂O₃-content is between 64 and 87% by weight.13. Composition according to claim 7, with an Fe₂O₃-content <0.5% byweight.
 14. Composition according to claim 7, in which the ZrO₂ contentis fashioned by a baddelyite-containing raw material.
 15. Compositionaccording to claim 7, in which the ZrO₂ content is made up by asinter-zirconium mullite, melt-zirconium mullite, sinter zirconiumcorundum, melt zirconium corundum or a ZAC material, separately or as amixture.
 16. Composition according to claim 7, in which the ZrO₂ contentis present in a grain size of between 1 and 4 mm.
 17. Compositionaccording to claim 7, with an MA-spinel content of between 5 and 25% byweight.
 18. Composition according to claim 7, in which at least one ofthe Cr₂O₃- and MgO containing components is present in a grain size of<0.2 mm.
 19. Composition according to claim 7, in which at least one ofthe Cr₂O₃ and MgO-containing components is present in a grain-sizefraction of <0.1 mm.
 20. Composition according to claim 7, in which upto 30% by weight is present as Al₂O₃ in a grain-size fraction of <0.1mm.
 21. Composition according to claim 7, in which Al₂O₃ is present in agrain-size fraction of >0.2 mm.